The equilibrium between blocked and closed states of muscle thin filaments is calcium sensitive (KB 0.3 at pCa 8.9 to >=10 at pCa 4.6) and depends on ionic strength and temperature.
The equilibrium between blocked and closed states of muscle thin filaments is sensitive to calcium, low ionic strength, and temperature.
We recently proposed a three-state model for the regulation of actomyosin interaction by tropomyosin and troponin (Tm.Tn). In this model, the thin filament exists in rapid equilibrium between the following three states: blocked, which cannot bind myosin significantly; closed, which can bind myosin weakly to form the A-state; open, which can bind both to form the A-state and isomerize to the strongly bound R-state. In this study, we demonstrate that the equilibrium between the blocked and closed states is calcium sensitive with an equilibrium constant, KB, of 0.3 and > or = 10 at pCa 8.9 and 4.6, respectively. The pCa dependence of KB is typical of that for calcium binding to thin filaments with a mid-point at pCa 5.6 and a Hill coefficient of 1.8. KB is independent of ionic strength over the range 0.4-0.06 M but increases dramatically below 0.05 M to > or = 10 at 0.01 M suggesting loss of the blocked state at low ionic strength. The blocked state also has reduced occupancy at high temperatures. KB, in the absence of calcium, increases from 0.2 at 5 degrees C to 0.6 at 40 degrees C.
Head et al. (Wed,) reported a other. The equilibrium between blocked and closed states of muscle thin filaments is calcium sensitive (KB 0.3 at pCa 8.9 to >=10 at pCa 4.6) and depends on ionic strength and temperature.